Pneumatic tire including toriodally continuous cells and method of producing same

ABSTRACT

A rubber composition is provided with a large friction force even on a body having, for example, an ice face of a low friction coefficient. The composition is constituted by a foamed rubber having a rationalized foaming state. The composition is applied to an upper-layer rubber part of a tread rubber constituting at least a ground contact face to enhance a gripping force on a frozen road surface or a snow covered road surface. The foamed rubber has continuous cells  6  each continuously extending substantially along a particular direction  4  and sealed with a resin protection membrane  5.  Also, a method is provided for producing such a tire.

TECHNICAL FIELD

This invention relates to a rubber composition of a foamed rubberproviding a large friction force even on a body having, for example, anice face of a low friction coefficient, and a pneumatic tire havingimproved steering stability performances such as braking performance,cornering performance and the like by applying this rubber compositionto an upper-layer rubber part of a tread rubber constituting at least aground contact face to enhance a gripping force on a frozen road surfaceor a snow covered road surface, particularly a winter-seasoned tire suchas a studless tire or the like as well as a method of producing such atire.

BACKGROUND ART

In the conventional pneumatic tires, particularly winter-seasoned tiressuch as studless tire or the like, there are proposed various methodsfor ensuring performances on ice or performances on snow.

As a way for increasing a friction coefficient on an ice-snow roadsurface, it is useful to microscopically form grooves by using a shortfiber-containing foamed rubber as a tread rubber and breaking away theshort fibers exposed on a surface of the tread accompanied with thewearing of the tread rubber during the running as described inJP-A-4-38207.

However, the microscopic grooves formed on the tread are liable to becrushed as a load applied to the tire becomes large, and hence theeffect of increasing the friction coefficient on the ice-snow roadsurface can not be sufficiently obtained.

Also, it is necessary that the short fibers are embedded in the foamedrubber at a state of extending in a straight line and substantially inparallel to a worn face of the tread when the tread is worn by therunning for rapidly breaking away the short fibers exposed on the wornface of the tread to microscopically form the grooves.

However, the short fibers are actually and frequently embedded in thefoamed rubber at a state of being curled through heat shrinkage in thevulcanization or pushing the fibers into a groove portion of a mold partor a sipe portion to bend in the tread rubber. In this case, even if thetread worn during the running, the short fibers not extendingsubstantially in parallel to the worn face of the tread can not easilybe broken away from the foamed rubber, so that the microscopic groovesas originally intended can not be formed efficiently.

As another way for ensuring the performances on ice or performances onsnow, it is useful to use a foamed rubber containing spherical closedcells in the tread as described in JP-A-62-283001.

That is, ice or snow on the ice-snow road surface is thawed to water byfriction heat when the tire tread contacts with the ice-snow roadsurface and this water forms a water membrane between the tread and theice-snow road surface, and the performances on ice or performances onsnow are deteriorated by this water membrane. In case of the tire usingthe above foamed rubber, the water membrane can be removed by the actionof irregularities on the tread formed by the closed cells, and hence itis attempted to improve the performances on ice or the performances onsnow.

However, the tire described in JP-A-62-283001 does not develop thesufficient water-removing effect because the irregularities formed onthe tread by the closed cell are very fine.

For this end, the applicant has made studies for enhancing the waterremoving effect on the ice-snow road surface and found that the waterremoving effect on the ice-snow road surface is conspicuous in pneumatictires provided with a tread having not only the spherical closed cellsbut also many continuous closed cells covered with a protection layer ofa resin, which has been proposed by an Application No. JP9700873 of aninternational application under PCT (international publication No.9734776).

In such a tire, it is considered that drainage paths can be particularlyformed by the continuous closed cells, whereby the water removing effectis enhanced.

However, the applicant has made further studies in order to more enhancethe water removing effect on the ice-snow road surface and revealed thatthere is room for improvement because it is hardly said that thesufficient drainage paths are formed in the tire as the length of thecontinuous closed cell is shorter than an arranging pitch of sipe orgroove disposed on the tread.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide a rubber composition givinga large friction force on a body having, for example, an ice face of alow friction coefficient by constituting with a foamed rubber having arationalized cell-existing state, and a pneumatic tire having improvedsteering stability performances such as braking performance, corneringperformance and the like by applying this rubber composition to at leastan upper-layer rubber part of a tread rubber constituting a groundcontact face to enhance a gripping force on a frozen road surface or asnow covered road surface, particularly a winter-seasoned tire such as astudless tire or the like as well as a method of producing such a tire.

In order to achieve the above object, the first invention is a rubbercomposition constituted with a foamed rubber, characterized in that thefoamed rubber has continuous cells each continuously extendingsubstantially along a particular direction and sealed with a resinprotection membrane.

Also, it is preferable that the foamed rubber comprises resin cordscontinuously extending in a thready form along a particular directionbefore vulcanization and a foaming agent, in which the resin cord ismelted in the vulcanization to develop a viscosity lower than that ofrubber constituting the foamed rubber and the foaming agent has aproperty of foaming in the vulcanization to produce gases, and thecontinuous cell is formed by sealing at least a part of gases producedfrom the foaming agent with the resin protection membrane made of theresin cord.

The second invention is a pneumatic tire comprising a tread rubber, atleast an upper-layer rubber part of which forming a ground contact facebeing made of a foamed rubber, characterized in that the upper-layerrubber part has toroidally continuous cells each continuously extendingsubstantially along a circumferential direction of the tire and sealedwith a resin protection membrane.

It is preferable that the foamed rubber comprises resin cords eachcontinuously extending in a thready form along the circumferentialdirection of the tire before vulcanization and a foaming agent, in whichthe resin cord is melted in the vulcanization to develop a viscositylower than that of rubber constituting the upper-layer rubber part andthe foaming agent has a property of foaming in the vulcanization toproduce gases, and the continuous cell is formed by sealing at least apart of gases produced from the foaming agent with the resin protectionmembrane made of the resin cord. Also, it is preferable that thecontinuous cells are arranged at plural stages in a thickness directionof the tread.

The resin cord is preferable to have a cord diameter of 10-100 μm and/orto have a melting point lower than a maximum vulcanization temperatureof the tread rubber.

The foamed rubber is preferable to have long cells of 0.5-5 mm in lengthin addition to the above continuous cells, in which these long cells arearranged so as to connect with the continuous cells in a network formand/or to have an existing ratio of all bubbles contained in itsinterior of 10-40% as a weight ratio and HD (hardness) of 38-58.

Moreover, the term “HD (hardness)” used herein means a scale reading ofa durometer hardness as measured at a testing temperature of 20° C. by atype-A durometer hardness testing machine defined in JIS K6253-1993.

The upper-layer rubber part is preferable to have a thicknesscorresponding to 30-70% of a thickness of the tread rubber.

The third invention is a method of producing a pneumatic tire whichcomprises covering one or plural resin cords arranged in parallel toeach other with a foamed rubber containing a foaming agent to form asheet-like member having a given width, winding and laminating thesheet-like member on a green case or a shaping drum along acircumferential direction of the green case or the shaping drum to forman upper-layer rubber part constituting a tread rubber and vulcanizing,during which the resin cord is melted to form a resin protectionmembrane and at least a part of gases produced by foaming of the foamingagent contained in the foamed rubber is sealed with the resulting resinprotection membrane to form toroidally continuous cells eachcontinuously extending substantially along a circumferential directionof the tire in the foamed rubber constituting the upper-layer rubberpart.

The sheet-like member is preferable to be formed by drawing one orplural resin cords and covering such drawn resin cords with the foamedrubber through an insulation system, or by sandwiching one or pluralresin cords arranged in parallel to each other between two foamed rubbersheets, or by surrounding and laminating one or plural resin cords withcord-shaped rubbers containing a foaming agent and covering with rubberthrough an inserter.

The fourth invention is a method of producing a pneumatic tire whichcomprises winding and laminating a narrow-width ribbon-shaped member ofat least one resin cord covered with a foamed rubber containing afoaming agent on a green case or a shaping drum along a circumferentialdirection of the green case or the shaping drum to form an upper-layerrubber part constituting a tread rubber and vulcanizing, during whichthe resin cord is melted to form a resin protection membrane and atleast a part of gases produced by foaming of the foaming agent containedin the foamed rubber is sealed with the resulting resin protectionmembrane to form toroidally continuous cells each continuously extendingsubstantially along a circumferential direction of the tire in thefoamed rubber constituting the upper-layer rubber part.

The ribbon-shaped member is preferable to be formed by drawing one orplural resin cords and covering such drawn resin cords with the foamedrubber through an insulation system, or by surrounding and laminatingone or plural resin cords with cord-shaped rubbers containing a foamingagent and covering with rubber through an inserter.

Also, at least the upper-layer rubber part of the tread rubber ispreferable to be formed by winding and laminating the ribbon-shapedmember so as to overlap widthwise end portions with each other everywinding while shifting in the widthwise direction of the tire.

The fifth invention is a method of producing a pneumatic tire whichcomprises winding and laminating a ribbon-shaped rubber sheet containinga foaming agent and resin cords on a green case or a shaping drum so asto arrange the resin cords at a given interval along a circumferentialdirection of the green case or the shaping drum to form an upper-layerrubber part constituting a tread rubber and vulcanizing, during whichthe resin cord is melted to form a resin protection membrane and atleast a part of gases produced by foaming of the foaming agent containedin the foamed rubber is sealed with the resulting resin protectionmembrane to form toroidally continuous cells each continuously extendingsubstantially along a circumferential direction of the tire in thefoamed rubber constituting the upper-layer rubber part.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a rubber composition according to thefirst invention.

FIG. 2 is a diagrammatic view illustrating a process of forming acontinuous cell in a foamed rubber constituting a rubber compositionaccording to the first invention.

FIG. 3 is a perspective view of a widthwise half-section of a pneumatictire according to the second invention together with a ground contactface of its tread portion.

FIG. 4 shows one of block land portions formed in the tread portion ofthe tire of FIG. 3, wherein (a) is a widthwise section view and (b) is aperspective view.

FIG. 5 is a view of another embodiment of the block land portion.

FIG. 6 is a schematic view illustrating a forming step of a sheet-likerubber member in the production method according to the third invention.

FIG. 7 is a schematic view illustrating another forming step of thesheet-like rubber member.

FIG. 8 is a schematic view illustrating a state of winding andlaminating a sheet-like rubber member on a shaping drum in theproduction method according to the third invention.

FIG. 9 is a schematic view illustrating a method of forming anupper-layer rubber part by using a narrow-width ribbon-shaped rubbermember.

FIG. 10 is a diagrammatic view illustrating a method of forming a longcell.

FIG. 11 is a schematic view illustrating a forming step of anarrow-width ribbon-shaped rubber member in the production methodaccording to the fourth invention.

FIG. 12 is a schematic view illustrating a method of forming aribbon-shaped member by using a resin cord and a cord-shaped rubber.

FIG. 13 is a schematic view illustrating a method of forming anupper-layer rubber part according to the fifth invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the first invention will be described with reference tothe drawings below.

FIG. 1 shows an example of a rubber composition 1 according to the firstinvention.

A main feature in the construction of the rubber composition 1 accordingto the first invention lies in the rationalization of a bubble existingstate in a foamed rubber constituting the rubber composition 1,concretely the formation of continuous cells 6 in the foamed rubber,each continuously extending substantially along a particular direction4, a longitudinal direction of the rubber composition 1 in FIG. 1 andsealed with a resin protection membrane 5. By adopting this constructioncan be formed drainage paths 8 when the continuous cells 6 are opened atthe ground contact face of the tread portion. As a result, when therubber composition 1 is moved in the longitudinal direction 4 whilecontacting with a body having, for example, an ice face of a lowfriction coefficient, a large friction force is always obtained by awater removing action of the drainage path 8.

The foamed rubber constituting the rubber composition 1 is preferable tocomprise resin cords 7 (see FIG. 2( a)) each continuously extending in athready form along the particular direction 4 before vulcanization and afoaming agent for forming the continuous cells 6, in which the resincord is melted in the vulcanization to develop a viscosity lower thanthat of rubber part constituting the foamed rubber and the foaming agenthas a property of foaming in the vulcanization to produce gases, and thecontinuous cell 6 is formed by sealing at least a part of gases producedfrom the foaming agent with the resin protection membrane made of theresin cord.

FIGS. 2( a)-(d) are views illustrating a process of forming thecontinuous cell 6 in the foamed rubber.

FIG. 2( a) shows a state of the foamed rubber when the heating of thefoamed rubber is started in a mold (not shown). The foaming agent in thefoamed rubber is gasified by the heating to start the generation ofbubbles 2′.

Then, as the foamed rubber is further heated to a higher temperature(e.g. a maximum vulcanization temperature), a resin cord 7 is melted (orsoftened) and a viscosity thereof is made lower than a viscosity of arubber part 3. As a result, the bubbles 2′ generated around theresulting molten resin 7′ are effectively entered into the molten resin7′ as shown in FIG. 2( b).

Thereafter, the bubbles 2′ entered into the molten resin 7′ areconnected to each other in a chain in the molten resin 7′ and finallysealed with a resin protection membrane 5 made of the molten resin 7′ toform a continuous cell 6. On the other hand, the bubbles 2′ generatedapart from the molten resin 7′ in the rubber part 3′ settle on theirpositions at a state of a spherical closed cell 2.

As a result, the spherical closed cells 2 and the continuous cells 6reinforced with the resin protection membranes 5 can be formed in thefoamed rubber constituting the rubber composition 1 of the firstinvention as shown in FIGS. 2( c) and 2(d).

An embodiment of the second invention will be described below.

FIG. 3 shows a widthwise left-half section of a pneumatic tire 10according to the second invention.

The illustrated pneumatic tire 10 is a studless tire and comprises atread portion 11, a pair of sidewall portions 12, a pair of beadportions 13, a carcass 15 of one ply toroidally extending between a pairof bead cores 14 each embedded in the bead portion 13 and fixed at bothend portions around the bead core, and a belt 16 of two rubberized cordlayers 16 a, 16 b arranged between a crown portion of the carcass andthe tread portion 11.

Also, the tread portion 11 is made of two layers, i.e. a lower-layerrubber part 17 made of a relatively hard rubber and an upper-layerrubber part 18 applied with the same foamed rubber as the rubbercomposition according to the first invention.

Further, plural tread grooves, i.e. tread grooves having pluralcircumferential grooves 20 extending along a circumferential direction19 of the tire and plural lateral grooves 21 intersecting therewith inFIG. 3 are arranged on the tread portion 11, whereby a tread landportion is divided into a plurality of block land parts 22. Further, aplurality of sipes 23 are arranged on each of the block land parts 22 asan edge component for ensuring performances on an ice-snow road surface.

Moreover, the term “tread groove” used herein includes all groovesarranged on the tread portion 11. For example, there are mentioned slantgrooves (not shown) inclined with respect to the circumferentialdirection 19 of the tire and the like in addition to the aforementionedcircumferential grooves 20 and lateral grooves 21.

A main feature in the construction of the pneumatic tire 10 according tothe second invention lies in that the upper-layer rubber part 18 is madeof the rubber composition 1 shown in FIG. 1, more concretely theupper-layer rubber part 18 has the continuous cells 6 each continuouslyextending substantially along the circumferential direction 19 of thetire and sealed with the resin protection membrane 5. By adopting thisconstruction can be enhanced the gripping force on a frozen road surfaceor a snow covered road surface to improve steering stabilityperformances such as braking performance, cornering performance and thelike.

FIGS. 4( a), (b) show one of the block land parts 22 constituting thetread portion worn by the running, wherein (a) is a widthwise sectionview and (b) is a perspective view. Moreover, the tire 10 has a toroidalform, so that the continuous cells 6 are toroidally extendedsubstantially along the circumferential direction 19 of the tire.

As seen from these figures, drainage paths 24 extending in thecircumferential direction 19 of the tire are always formed on the groundcontact face of the tread portion in the tire 10 by opening thecontinuous cells 6 each sealed with the resin protection membrane 5through the wearing of the tread. Furthermore, these drainage paths 24communicate with the plural lateral grooves 21 defining the block landpart and also communicate with the plural sipes 23 arranged in the sameblock land part, so that there can be obtained good water removingeffect.

It is preferable that the foamed rubber comprises resin cords 7continuously extending in a thready form along the circumferentialdirection 19 of the tire before vulcanization and a foaming agent forforming the continuous cells 6, in which the resin cord 7 is melted inthe vulcanization to develop a viscosity lower than that of rubber part3′ constituting the foamed rubber and the foaming agent has a propertyof foaming in the vulcanization to produce gases, and the continuouscell 6 is formed by sealing at least a part of gases produced from thefoaming agent with the resin protection membrane 5 made from the resincord 7 (see FIG. 2).

Also, it is preferable that the resin cords 7 are arranged in a densityof about one cord/1 mm for the purpose of removing water in thecircumferential direction 19 of the tire. In addition, a cord diameterof the resin cord is preferable to be 10-100 μm in view of thesecurement of actual contact area.

Further, the continuous cells 6 are preferable to be arranged regularlyor randomly at plural stages in not only the widthwise direction of thetire but also the thickness direction of the tread as shown in FIGS. 4(a) and (b). By arranging the continuous cells 6 as mentioned above, thedrainage paths 24 can be always existent on the ground contact face ofthe tread portion in the wearing.

The upper-layer rubber part 18 is preferable to have long cells 26 of0.5-5 mm in length in addition to the above continuous cells 6, in whichthese long cells 26 are arranged so as to connect with the continuouscells 6 in a network form as shown in FIG. 5 in view of the shorteningof the time required in the drainage or the increase of the drainageefficiency.

Moreover, as a way for forming the long cells 26 in the upper-layerrubber part 18, it is preferable to use a method wherein long resinpieces 37 are kneaded with an uncured rubber composition 38 in anextruder 39 and a kneaded mass is extruded through a die 40 of theextruder into a given shape as shown in FIG. 10 and then the resultingsheet-shaped rubber member 41 is used as a starting material for theupper-layer rubber part 18 and thereafter foamed through vulcanization.

The resin cord is required to have a melting point lower than a maximumvulcanization temperature of the tread rubber itself because it formsthe resin protection membrane 5 sealing the continuous cell 6 therein ina tire product. The melting point is made preferably by 10° C. or less,particularly 30° C. or less than the maximum vulcanization temperature.

That is, when the difference between the melting point of the resin cord7 and the maximum vulcanization temperature is too small, the melting ofthe resin cord 7 starts at a last stage of the vulcanization and therubber part already takes a greater amount of gases therein to promotecross-linking reaction and hence an amount of gas captured in the moltenresin cord becomes small and the formation of the continuous cell 6 isdifficult, while when the difference between the melting point of theresin cord 7 and the maximum vulcanization temperature is too large(concretely the melting point of the resin cord is low), the resin cords7 are melted by heat in the kneading of rubber and the fusion bondingbetween the molten resin cords 7′ is caused at this kneading stage andit is difficult to extend the continuous cells 6 along thecircumferential direction 19 of the tire.

The term “maximum vulcanization temperature of tread rubber itself” usedherein means a maximum temperature of the tread portion in thevulcanization inside a mold for a period ranging up to the cooling ofthe tire outside the mold.

Also, the upper-layer rubber part 18 is preferable to have a thicknesscorresponding to 30-70%, more preferably 40-55% of a thickness of thetread rubber. When the thickness of the upper-layer rubber part 18 isless than 30% of the thickness of the tread rubber, the foamed rubberconstituting the upper-layer rubber part 18 becomes not existent at thelast use stage of the tire and it is difficult to ensure the stableperformances on ice-snow road in the service life of the tire, whilewhen it exceeds 70%, the rigidity of the tread portion is apt to belacking and there is caused a fear that the steering stability in usualrunning (running on dry road surface) can not be sufficiently ensured.

Furthermore, the foamed rubber constituting the upper-layer rubber part18 is preferable that an existing ratio of all bubbles contained thereinis a range of 10-40% as a weight ratio and HD is 38-58. When theexisting ratio of all bubbles contained in the interior of the foamedrubber is less than 10% as a weight ratio, the drainage efficiencylowers, while when it exceeds 40%, the wear resistance and theperformances on ordinary road are deteriorated. Also, when HD of thefoamed rubber is less than 38, the wear resistance and the performanceson ordinary road are deteriorated, while when it exceeds 58, the tirehardly follows to micro-irregularities on ice road.

In addition, the continuous cell 6 is preferable to have a sectionaldiameter of 10-200 μm. When the sectional diameter is less than 10 μm,the drainage efficiency is insufficient, while it exceeds 200 μm, theactual contact area lowers and hence the friction coefficient on icelowers.

The production method of the pneumatic tire according to the third tofifth inventions will be described below.

At first, the method of forming the upper-layer rubber part 18 accordingto the third invention is explained. In the third invention, one orplural resin cords are arranged in parallel and covered with foamedrubber containing a foaming agent to form a sheet-like member having agiven width. The term “given width” used herein means the same width asa tread rubber.

FIG. 6( a) shows an example of a method of forming a sheet-like member30.

This figure shows a case that one or plural resin cords 7 are drawn outfrom a respective bobbin(s) 31 and the drawn resin cords 7 are arrangedin parallel and covered with foamed rubbers 33 a, 33 b extruded throughtwo rubber extruders 32 a, 32 b through an insulation system, which isthen passed through an inserter 34 to integrally unite the resin cords 7and the foamed rubbers 33 a, 33 b with each other to thereby form asheet-like member 30. Also, the sheet-like member 30 may be formed bycovering the resin cords 7 through a single rubber extruder. Theformation method of the sheet-like member 30 is not limited to the aboveand various formation methods can be adopted.

For example, when the resin cord is made from a frangible material orthe like, as shown in FIG. 7, the sheet-like member 30 is preferable tobe formed by placing resin cords 7A, 7B, 7C . . . having a length equalto a peripheral length of a green case extruded through an extrudingmachine 42 on a foamed rubber sheet 33 b side by side and placinganother foamed rubber sheet 33 a thereon to sandwich the resin cords 7A,7B, 7C . . . between the foamed rubber sheets 33 a, 33 b.

Then, the sheet-like member 30 is wound and laminated on a green case ora shaping drum, which is previously wound and laminated with a rubbersheet constituting a lower-layer rubber part 17, a shaping drum 35 inFIG. 8 so as to extend the resin cords 7 embedded in the sheet-likemember 30 along a circumferential direction of the shaping drum 35 tothereby form the upper-layer rubber part 18.

The method of forming the upper-layer rubber part 18 in the productionmethod of the fourth invention will be described below. In the fourthinvention, the upper-layer rubber part 18 is formed by using anarrow-width ribbon-shaped member obtained by covering at least oneresin cord with a foamed rubber containing a foaming agent.

FIG. 11 is a view illustrating an example of the method of forming aribbon-shaped member 43.

In this figure, one resin cord 7 is covered with a foamed rubberextruded from a rubber extruder 32 through an insulation system andpassed through an inserter 34 to cover the resin cord 7 with the foamedrubber to thereby form a ribbon-shaped member 43.

Also, the ribbon-shaped member 43 may be formed by laminating pluralcord-shaped rubbers 44 containing a foaming agent around one resin cord7 and passing through the inserter to cover the resin cord 7 with rubberas shown in FIG. 12( a), or by laminating plural cord-shaped rubbers 44containing a foaming agent around each of plural resin cords 7 andpassing through the inserter to cover the resin cords 7 with rubber asshown in FIG. 12( b).

Then, the ribbon-shaped member 43 is wound and laminated on a green caseor a shaping drum, which is previously wound and laminated with a rubbersheet constituting a lower-layer rubber part 17, so as to extend theresin cords 7 embedded in the ribbon-shaped member 43 along acircumferential direction of the shaping drum to thereby form theupper-layer rubber part 18.

In the fourth invention, as shown in FIG. 9, at least the upper-layerrubber part 18 of the tread rubber may be formed by winding andlaminating the ribbon-shaped member 43 so as to overlap width endportions with each other every winding while shifting in the widthwisedirection 36 of the tire. According to this forming method, thecontinuous cells 6 can be effectively arranged in not only the widthwisedirection of the tread but also the thickness direction of the tread, sothat as the tire is worn, the drainage paths 24 formed by opening thecontinuous cells 6 can be always existent in the ground contact face ofthe tread portion 11 and hence the lowering of the performances onice-snow during the wearing can be suppressed.

Such an effect can also be obtained even by using the sheet-like memberaccording to the third invention instead of the ribbon-shaped member.

The method of forming the upper-layer rubber part 18 according to thefifth invention will be described below.

As shown in FIG. 13( a), a cord-shaped rubber 44 is wound on thelower-layer rubber part 17 while shifting in the widthwise direction 36of the tire to form a single rubber layer 45. Then, plural resin cords 7and the cord-shaped rubber 44 are wound on t he single rubber layer 45so as to arrange the resin cords 7 at given intervals to form acord-rubber layer 46. The single rubber layer 45 and the cord-rubberlayer 46 are successively laminated to obtain a given thickness tothereby form the upper-layer rubber part 18. Moreover, this figure showsa case that the single rubber layer 45 is formed with the pluralcord-shaped rubbers 44, but the single rubber layer 45 may beconstituted with a single wide-width rubber sheet as shown in FIG. 13(b).

When the upper-layer rubber part 18 formed according to any one of thethird to fifth inventions is used to form a green tire and thensubjected to vulcanization, the resin cords 7 in the foamed rubberconstituting the upper-layer rubber part of the tire tread are melted toform the resin protection membrane 5 and at the same time at least apart of gases produced by foaming the foaming agent contained in thefoamed rubber is sealed with the rein protection membrane 5, wherebythere can be formed the continuous cells 6 each continuously extendingsubstantially along the circumferential direction 19 of the tire.

Although the above is merely described with respect to the embodiment ofthe invention, various modifications can be conducted within the scopeof the invention.

Then, the studless tire according to the second invention is prepared bythe method according to the third invention applying the rubbercomposition according to the first invention to the upper-layer rubberpart and the performances thereof are evaluated below.

EXAMPLES

Tires of Examples 1-9 are studless radial tires each having a halfsection in a widthwise direction of the tire shown in FIG. 3 and a tiresize of 205/65R15, in which continuous cells are arranged in anupper-layer rubber part at 5 stages and 30 rows with an interval of 1 mmand a thickness of the upper-layer rubber part is 45% of a thickness ofa tread rubber and an existing ratio of all bubbles contained in aninterior of a foamed rubber constituting the upper-layer rubber part asa weight ratio and HD of the foamed rubber are shown in Table 1.Moreover, the upper-layer rubber part is formed by winding andlaminating a narrow-width ribbon-shaped member having a thickness of 0.5mm and a width of 10 mm so as to overlap widthwise end portion with eachother every winding while shifting in the widthwise direction of thetire as shown in FIG. 9. Also, the resin cords are arranged in theribbon-shaped member at a density of 1.5 cords/1 mm. The resin cordembedded in the ribbon-shaped member before vulcanization has a corddiameter of 30 μm and a melting point of 140° C., which is lower byabout 30° C. than a maximum vulcanization temperature of the treadrubber itself.

Moreover, the other construction of the tire is substantially the sameas in the ordinary pneumatic radial tire for a passenger car.

Conventional Example

The tire of Conventional Example is the same as the tire of Example 1except that the foamed rubber constituting the upper-layer rubber parthas an expansion ratio of 26% as a weight ratio of rubber, HD of 48 anda thickness corresponding to 45% of a whole of a tread and thecontinuous cells are not formed.

(Test Method)

Each of the above tires is assembled into a rim of 6.5J×15 to form atire-rim assembly, which is mounted onto a passenger car. Such apassenger car is run on various road surfaces (dry road surface, wetroad surface, frozen road surface, snow covered road surface) toevaluate braking performances on the frozen road surface and snowcovered road surface, and steering stability performances on the dryroad surface and wet road surface. In this case, a tire internalpressure is 190 kPa in front and rear tires, and a load corresponds totwo crewmen.

The braking performance is evaluated by measuring a stopping distancewhen quick braking (full braking) is applied from a speed of 20 km/h onthe frozen road surface or 40 km/h on the snow covered road surface atan OFF state of ABS (anti-lock braking system).

The steering stability performance is evaluated by measuring an averagetime every running when the tire is run on a circuit course 10 times.

These evaluation results are also shown in Table 1. Moreover, thenumerical values in Table 1 is represented by an index on the basis thatthe conventional example is 100, in which the larger the numericalvalue, the better the performance.

TABLE 1 Conventional Example Example 1 2 3 4 5 6 7 8 9 HD 48 48 43 53 3565 48 48 48 48 Weight ratio of 26% 26% 26% 26% 26% 26% 16% 36% 5% 50%foamed rubber A. Braking 100 110 112 108 115 103 105 115 100 120performance *1 B. Braking 100 107 109 105 112 100 102 112 97 117performance *2 C. Steering stability 100 102 101 103 99 105 104 100 10697 performance *3 D. Steering stability 100 101 100 102 98 104 103 99105 96 performance *4 (Note) *1: on frozen road surface *2: on snowcovered road surface *3: on dry road surface *4: on wet road surface

As seen from the evaluation results of Table 1, the example tires areequal level in the steering stability performance on the dry and wetroad surfaces but are considerably excellent in the braking performanceson the frozen and snow covered road surfaces as compared with theconventional tire.

INDUSTRIAL APPLICABILITY

According to the invention, there can be provided a rubber compositionproviding a large friction force even on a body having, for example, anice face of a low friction coefficient by constituting with a foamedrubber having a rationalized foaming state, and a pneumatic tire,particularly a winter-seasoned tire such as a studless tire havingimproved steering stability performances such as braking performance,cornering performance and the like by applying this rubber compositionto an upper-layer rubber part of a tread rubber constituting at least aground contact face to enhance a gripping force on a frozen road surfaceor a snow covered road surface as well as a method of producing such atire.

1. A pneumatic tire comprising a tread rubber in a tread portionincluding grooves and sipes, at least an upper-layer rubber part ofwhich forming a ground contact face being made of a foamed rubber,characterized in that the upper-layer rubber part has toroidallycontinuous cells each continuously extending substantially along acircumferential direction of the tire and sealed with a resin protectionmembrane and having a length longer than an arranging pitch of the sipesor grooves and a sectional diameter of 10-200 μm.
 2. A pneumatic tireaccording to claim 1, wherein the continuous cells are arranged atplural stages in a thickness direction of a tread.
 3. A pneumatic tireaccording to claim 1, wherein the foamed rubber comprises resin cordseach continuously extending in a thready form along the circumferentialdirection of the tire before vulcanization and a foaming agent, in whichthe resin cord is melted in the vulcanization to develop a viscositylower than that of rubber constituting the upper-layer rubber part andthe foaming agent has a property of foaming in the vulcanization toproduce gases, and the continuous cell is formed by sealing at least apart of gases produced from the foaming agent with the resin protectionmembrane made of the resin cord.
 4. A pneumatic tire according to claim3, wherein the resin cord has a cord diameter of 10-100 μm.
 5. Apneumatic tire according to claim 3, wherein the resin cord has amelting point lower than a maximum vulcanization temperature of thetread rubber.
 6. A pneumatic tire according to claim 1, wherein thefoamed rubber has long cells of 0.5-5 mm in length in addition to thecontinuous cells, in which these long cells are arranged so as toconnect with the continuous cells in a network form.
 7. A pneumatic tireaccording to claim 1, wherein the foamed rubber has an existing ratio ofall bubbles contained in its interior of 10-40% as a weight ratio of thefoamed and unfoamed rubber, and HD (hardness) of 38-58.
 8. A pneumatictire according to claim 1, wherein the upper-layer rubber part has athickness corresponding to 30-70% of a thickness of the tread rubber. 9.A method of producing a pneumatic tire which comprises covering one orplural resin cords arranged in parallel to each other with a foamablerubber containing a foaming agent to form a sheet-like member having agiven width, winding and laminating the sheet-like member on a greencase or a shaping drum along a circumferential direction of the greencase or the shaping drum to form an upper-layer rubber part constitutinga tread rubber and vulcanizing to form foamed tread rubber includinggrooves and sipes, during which the resin cord is melted to form a resinprotection membrane and at least a part of gases produced by foaming ofthe foaming agent contained in the foamable rubber is sealed with theresulting resin protection membrane to form toroidally continuous cellseach continuously extending substantially along a circumferentialdirection of the tire in the foamed rubber constituting the upper-layerrubber part, the cells having a length longer than an arranging pitch ofthe sipes or grooves and a sectional diameter of 10-200 μm.
 10. A methodof producing a pneumatic tire which comprises winding and laminating anarrow-width ribbon-shaped member of at least one resin cord coveredwith a foamable rubber containing a foaming agent on a green case or ashaping drum along a circumferential direction of the green case or theshaping drum to form an upper-layer rubber part constituting a treadrubber and vulcanizing to form foamed tread rubber including grooves andsipes, during which the resin cord is melted to form a resin protectionmembrane and at least a part of gases produced by foaming of the foamingagent contained in the foamable rubber is sealed with the resultingresin protection membrane to form toroidally continuous cells eachcontinuously extending substantially along a circumferential directionof the tire in the foamed rubber constituting the upper-layer rubberpart, the cells having a length longer than an arranging pitch of thesipes or grooves and a sectional diameter of 10-200 μm.
 11. A method ofproducing a pneumatic tire according to claim 9, wherein the member isfonned by drawing one or plural resin cords and covering such drawnresin cords with the foarnable rubber through an insulation system. 12.A method of producing a pneumatic tire according to claim 9, wherein themember is formed at least in part by laminating one or plural resincords with narrow strips of rubber containing a foaming agent.
 13. Amethod of producing a pneumatic tire according to claim 9, wherein themember is formed by sandwiching one or plural resin cords arranged inparallel to each other between two foamable rubber sheets.
 14. A methodof producing a pneumatic tire according to claim 10, wherein at least anupper-layer rubber part of the tread rubber is formed by winding andlaminating the ribbon-shaped member so as to overlap widthwise endportions with each other every winding while shifting in a widthwisedirection of the tire.
 15. A method of producing a pneumatic tire whichcomprises winding and laminating a ribbon-shaped rubber sheet containinga foaming agent and resin cords on a green case or a shaping drum so asto arrange the resin cords at a given interval along a circumferentialdirection of the green case or the shaping drum to form an upper-layerrubber part constituting a tread rubber and vulcanizing to form foamedtread rubber including grooves and sipes, during which the resin cord ismelted to form a resin protection membrane and at least a part of gasesproduced by foaming of the foaming agent contained in the foamablerubber is sealed with the resulting resin protection membrane to formtoroidally continuous cells each continuously extending substantiallyalong a circumferential direction of the tire in the foamed rubberconstituting the upper-layer rubber part, the cells having a lengthlonger than an arranging pitch of the sipes or grooves and a sectionaldiameter of 10-200 μm.